The present invention relates generally to electrolyte materials. More particularly, the present invention relates to solid polymer electrolyte materials that are ionically conductive, mechanically robust, and manufacturable by conventional polymer processing methods. Merely by way of illustration, an exemplary polymer electrolyte material has an elastic modulus in excess of 1×106 Pa at 25 degrees C. and is characterized by an ionic conductivity of at least 1×10−5 Scm−1 at 90 degrees C. Many uses are contemplated for the solid polymer electrolyte materials. By way of example, the present invention can be applied to improve Li-based batteries by means of enabling higher energy density, better thermal and environmental stability, lower rates of self-discharge, enhanced safety, lower manufacturing costs, and novel form factors.
The demand for rechargeable batteries has grown by leaps and bounds as the global demand for technological products such as cellular phones, laptop computers and other consumer electronic products has escalated. In addition, interest in rechargeable batteries has been fueled by current efforts to develop green technologies such as electrical-grid load leveling devices and electrically-powered vehicles, which are creating an immense potential market for rechargeable batteries with high energy densities.
Li-ion batteries represent one of the most popular types of rechargeable batteries for portable electronics. Li-ion batteries offer high energy and power densities, slow loss of charge when not in use, and they do not suffer from memory effects. Because of many of their benefits, including their high energy density, Li-ion batteries have also been used increasingly in defense, aerospace, back-up storage, and transportation applications.
The electrolyte is an important part of a typical Li-ion rechargeable battery. Traditional Li-ion rechargeable batteries have employed liquid electrolytes. An exemplary liquid electrolyte in Li-ion batteries consists of lithium-salt electrolytes, such as LiPF6, LiBF4, or LiClO4, and organic solvents, such as an alkyl carbonate. During discharging, the electrolyte may serve as a simple medium for ion flow between the electrodes, as a negative electrode material is oxidized, producing electrons, and a positive electrode material is reduced, consuming electrons. These electrons constitute the current flow in an external circuit.
While liquid electrolytes dominate current Li-based technologies, solid electrolytes may constitute the next wave of advances for Li-based batteries. The lithium solid polymer electrolyte rechargeable battery is an especially attractive technology for Li-ion batteries because, among other benefits, the solid polymer electrolyte exhibits high thermal stability, low rates of self-discharge, stable operation over a wide range of environmental conditions, enhanced safety, flexibility in battery configuration, minimal environmental impacts, and low materials and processing costs. Moreover, solid polymer electrolytes may enable the use of lithium metal anodes, which offer higher energy densities than traditional lithium ion anodes.
Lithium batteries with solid electrolytes function as follows. During charging, a voltage applied between the electrodes of a battery causes lithium ions and electrons to be withdrawn from lithium hosts at the battery's positive electrode. Lithium ions flowing from the positive electrode to the battery's negative electrode through a polymer electrolyte are reduced at the negative electrode. During discharge, the opposite reaction occurs. Lithium ions and electrons are allowed to re-enter lithium hosts at the positive electrode as lithium is oxidized at the negative electrode. This energetically favorable, spontaneous process converts chemically stored energy into electrical power that an external device can use.
Polymeric electrolytes have been the subject of academic and commercial battery research for several years. Polymer electrolytes have been of exceptional interest partly due to their low reactivity with lithium and potential to act as a barrier to the formation of metallic lithium filaments (or dendrites) upon cycling.
According to one example, polymer electrolytes are formed by incorporating lithium salts into appropriate polymers to allow for the creation of electronically insulating media that are ionically conductive. Such a polymer offers the potential to act both as a solid state electrolyte and separator in primary or secondary batteries. Such a polymer can form solid state batteries that exhibit high thermal stability, low rates of self-discharge, stable operation over a wide range of environmental conditions, enhanced safety, and higher energy densities as compared with conventional liquid-electrolyte batteries.
Despite their many advantages, the adoption of polymer electrolytes has been curbed by the inability to develop an electrolyte that exhibits both high ionic conductivity and good mechanical properties. This difficulty arises because high ionic conductivity, according to standard mechanisms, calls for high polymer chain mobility. But high polymer chain mobility, according to standard mechanisms, tends to produce mechanically soft polymers.
As an example, a prototypical polymer electrolyte is one composed of polyethylene oxide (PEO)/salt mixtures. PEO generally offers good mechanical properties at room temperature. However, PEO is also largely crystalline at room temperature. The crystalline structure generally restricts chain mobility, reducing conductivity. Operating PEO electrolytes at high temperature (i.e., above the polymer's melting point) solves the conductivity problem by increasing chain mobility and hence improving ionic conductivity. However, the increased conductivity comes at a cost in terms of deterioration of the material's mechanical properties. At higher temperatures, the polymer no longer behaves as a solid.
In general, attempts to stiffen PEO, such as through addition of hard colloidal particles, increasing molecular weight, or cross-linking, have been found to also cause reduced ionic conductivity. Similarly, attempts to increase the conductivity of PEO, such as through addition of low molecular weight plasticizers, have led deterioration of mechanical properties.
Therefore, there has been and is still a strong need for a polymeric electrolyte material with high ionic conductivity and mechanical stability where the material is amenable to standard high-throughput polymer processing methods.